1. Field of the Invention
The present invention relates to a nonaqueous electrolyte secondary battery.
2. Description of Related Art
In a nonaqueous electrolyte secondary battery, further improvement in energy density has been studied in an effort to improve performance. The improvement in energy density can be realized, for example, by increasing the action potential of a positive electrode to be higher than that in the related art. However, in a case where the action potential of a positive electrode is set to be higher than that of a general nonaqueous electrolyte secondary battery, for example, to be 4.3 V or higher vs. lithium metal, the durability of a battery may deteriorate significantly. Therefore, as a result of thorough investigation, the present inventors conceived a nonaqueous electrolyte secondary battery that includes an inorganic phosphate compound (a phosphate and/or a pyrophosphate) in a positive electrode (refer to Japanese Patent Application Publication No. 2014-103098 (JP 2014-103098 A)). According to the above-described configuration, the elution of a transition metal element from a positive electrode active material can be prevented, and the durability of a battery can be improved.
The present inventors repeated additional evaluation and investigation on the above-described technique. As a result, it was found that there is room for further improvement in a case where the above-described technique is applied to a battery which is used for an aspect where high-rate charging and discharging is repeated. That is, in general, the electron conductivity of an inorganic phosphate compound is extremely low. Therefore, when a positive electrode contains an inorganic phosphate compound, the durability is improved; on the contrary, the resistance may increase. As a result, input and output characteristics may deteriorate.